It is proposed to study specific biochemical steps in the conversion of cholesterol to the two primary bile acids, cholic acid and chenodeoxycholic acid in experimental animals (rat, guinea pig and rabbit) in vivo and to elucidate the specific intracellular sites of bile acid synthesis, in vitro. Emphasis will be placed on the mechanism of degradation of the cholesterol side-chain, i.e., the sites, sequences and quantitative significance of the various hydroxylation reactions and loss of the terminal isopropyl group. It is further proposed to carry over only the most promising and feasible animal experiments to man and to study normolipidemic subjects and selected patients with documented inherited disorders of bile acid synthesis. Emphasis will be placed on re-evaluation of the major pathways for both cholic and chenodeoxycholic acid in vivo and comparison of localization and properties of key enzymes with those in the animal studies.